1. Field of the Invention
The present invention relates to an inexpensive, high-strength powder metallurgy titanium alloy and to a method of producing the same.
2. Description of the Related Art
Titanium alloys have a higher specific strength and specific toughness than ultrahigh-strength steel and high-strength aluminum alloys. On the other hand, they are poor in yield because of their difficulties involved in melting, casting, and machining. This has led one to believe that they are unsuitable for mass-produced parts.
It seems possible to overcome these difficulties by employing powder metallurgy, which permits the production of parts that need only a few finishing steps. Of many powder metallurgy methods, a promising one is the mixed powder method which involves the mixing of pure titanium powder and strengthening powder, which is followed by compacting and sintering. This method offers several advantages, including inexpensive raw material powder, high yields, and simple production process, which will lead to a considerable cost saving. The conventional mixed powder method, however, suffers from a disadvantage that it gives rise to a sintered titanium alloy which is as poor as cast materials in mechanical properties, especially fatigue strength. Therefore, it can be applied to the production of small components (such as nuts, fasteners, and filters) and missile parts (such as dome housings and gyroscope gimbals) which do not need high fatigue strength, but it cannot be applied to the production of important parts which need high fatigue strength.
In order to address this problem, various attempts have recently been made to improve fatigue strength by using a ultrahigh-purity titanium powder as a raw material and carrying out hot isostatic pressing and heat treatment after sintering.
Among the improved methods is "Production of titanium alloys by the mixed powder method" proposed in Japanese Patent Publication No. 29864/1989. This method consists of mixing the constituent metal powders, compacting the mixture, vacuum-sintering the compact, thereby forming a sintered titanium alloy, quenching the sintered compact from the .beta.-transus temperature (which is far below the sintering temperature) to room temperature or below, and finally heating the quenched compact under pressure at a temperature between 800.degree. C. and the .beta.-transus temperature (at which the .alpha.+.beta. two-phase region exists), thereby removing residual pores. In other words, this method involves the strengthening of sintered titanium alloy by the subtle combination of hot isostatic pressing and heat treatment. Therefore, this method, which is the mixed powder method, provides a sintered titanium alloy similar to that obtained by the alloyed powder method. The resulting sintered titanium alloy has a fine, homogeneous microstructure and a high fatigue strength.
Both the alloyed powder method and the mixed powder method provide their respective .alpha.+.beta. alloys through hot isostatic pressing. However, the .alpha.+.beta. alloys differ in microstructure because the sintered compacts before hot isostatic pressing differ in microstructure. The alloyed powder method employs an alloy powder prepared by quenching, which is subsequently solidified as such at a temperature below the .beta.-transus temperature. Therefore, the tempering of martensite takes place during hot isostatic pressing, giving rise to the fine .alpha.+.beta. microstructure. By contrast, the mixed powder method provides a sintered titanium alloy which has a coarse acicular .alpha.-phase due to .beta./.alpha. transformation which takes place in the cooling step which follows sintering. This sintered titanium alloy remains unchanged in microstructure even after hot isostatic pressing at a temperature below the .beta.-transus temperature.
According to Japanese Patent Publication No. 29864/1989 cited above, this disadvantage is eliminated by performing .beta.-quenching after sintering, thereby changing the microstructure into the fine martensite, and then performing hot isostatic pressing. This process is greatly affected by residual pores. The sintered compact contains residual pores which account for about 5 vol %. They completely suppress the grain growth of .beta.-phase during the solution treatment which is performed in the .beta.-region. Therefore, quenching provides a fine martensite microstructure and the subsequent hot isostatic pressing in the .alpha.+.beta. two-phase region forms the fine .alpha.-phase with a small aspect ratio similar to that provided by the alloyed powder method. The method disclosed in Japanese Patent Publication No. 29864/1989 cited above employs a titanium powder with an extremely low chlorine content which leaves no residual pores at all, so that the resulting titanium alloy is comparable in fatigue strength to that obtained by the alloyed powder method.
According to the method disclosed in Japanese Patent Publication No. 29864/1989 cited above, it is possible to improve the mechanical properties of sintered titanium alloys by the combination of hot isostatic pressing and heat treatment. This method, however, has a disadvantage of needing an expensive extra low chlorine powder as a raw material and needing the hot isostatic pressing and heat treatment after sintering. This disadvantage, which inevitably leads to a marked cost increase, makes the method unsuitable for the mass production of cheap automotive parts and the like.
Another method of producing a sintered titanium alloy is disclosed in Japanese Patent Publication No. 50172/1990 entitled "Method for producing a high-density sintered titanium alloy". This method involves the steps of (a) preparing alloy-forming particles (0.5-20 .mu.m in average particle diameter) by using a pulverizer capable of providing high energy, (b) mixing the alloy-forming particles with titanium base metal particles (40-177 .mu.m in average particle diameter), thereby forming a powder mixture in which the titanium base metal powder accounts for 70-95%, with the balance being the alloy-forming particles, and (c) forming the powder mixture into a green compact and sintering it at a temperature below that at which the liquid phase appears. It is claimed in this disclosure that the mechanical energy given during disintegration is accumulated as strain energy in the powder and this strain energy promotes sintering, giving rise to a relative density higher than 99%, without requiring any other steps than compacting and sintering, and that the resulting sintered alloy has much better mechanical properties as compared with that obtained by the ordinary method.
However, the above-mentioned claim is not convincing because the ordinary mother alloy such as Al.sub.3 V is hardly capable of plastic deformation and hence incapable of accumulating in the powder during disintegration so much energy as to promote sintering. The densification achieved by this method is due to the fact that the mother alloy powder decreases in average particle diameter and increases in surface energy in the pulverizing step. The promotion of sintering by pulverization is a known fact, and the fatigue strength attained by this method is 40 kg/mm.sup.2 at the highest (even when the compacting pressure is increased) although it is higher than that attained by the conventional method.
Japanese Patent Laid-open No. 130732/1988 discloses "Method for producing a high-density sintered titanium alloy", which involves the mixing of a titanium powder or titanium alloy powder composed of 25 wt % or more particles finer than 325 mesh with an alloying powder finer than 325 mesh in a prescribed ratio, which is followed by mechanical pulverization, compacting, and sintering. According to this disclosure, the mixture of a titanium powder and a mother alloy powder is pulverized in a high-energy ball mill so that the finely ground particles mechanically aggregate to form larger particles, and the thus prepared powder yields a high-density sintered body after compacting and sintering.
The copulverization of a titanium powder and a mother alloy powder, as disclosed in Japanese Patent Laid-open 130732/1988 cited above, needs a very large amount of energy to greatly deform and pulverize the highly ductile titanium powder. This leads to a disadvantage that the greatly deformed titanium powder undergoes marked work hardening and hence decreases in compressibility. This in turn makes it necessary to increase the forming pressure to such a level which is by far higher than that required in the ordinary process, in order to increase the density of the compact. It is known that intensive working following pulverization brings about aggregation, and the aggregate powder has such a simple shape that it is very poor in forming performance. An additional disadvantage of this method is that the active titanium powder inevitably takes up a large amount of oxygen in the pulverizing step. The absorbed oxygen has an adverse effect on mechanical properties, especially ductility, of the sintered titanium alloy.
The above-mentioned prior arts are based on the known titanium alloys developed for the ingot metallurgy, and hence they disclose nothing about the titanium alloys prepared by utilizing the feature of the mixed powder method.
In order to improve the heat resistance, stiffness, and wear resistance of sintered titanium alloys, a composite material has recent been developed which contains hard particles dispersed therein. The dispersed particles are those of TiC, TiN, SiC, and TiB.sub.2. An example of the titanium-based composite material is disclosed in U.S. Pat. No. 4,731,115, entitled "Titanium carbide/titanium alloy composite and process for powder metal cladding". This disclosure concerns a titanium-based composite material containing TiC particles dispersed therein, which is produced from a titanium powder, mother alloy powder for solid-solution hardening, and TiC powder, by mixing, forming, sintering, and hot isostatic pressing. This disclosure also concerns a laminate of powder alloy. It is claimed that the composite material thus obtained has a high Young's modulus and good wear resistance.
The composite material disclosed in U.S. Pat. No. 4,731,115 cited above has a disadvantage of high production cost resulting from hot isostatic pressing. Another disadvantage includes decreased ductility and coarse grains. The decreased ductility is due to the fact that the titanium alloy matrix dissolves a considerable amount of carbon although TiC particles are less reactive to the matrix than SiC as a reinforcing fiber for titanium-based FRM. The coarse grains result from the Ostwald Ripening which is enhanced by incoherent interface between TiC particles and the titanium alloy matrix and the tendency of carbon toward dissolution in the matrix. In addition, this composite material has to be consolidated at a low temperature (with low-temperature, high-pressure hot isostatic pressing) to prevent the particle/matrix reaction and grain growth. Any violation of this condition will result in a composite material which has a high stiffness but is poor in ductility. It can be said, therefore, that TiC particles are not necessarily the best although they are by far superior to SiC particles in compatibility with the titanium alloy.
Japanese Patent Laid-open No. 129330/1990 entitled "Highly wear resistant titanium alloy material" discloses a titanium-based composite material containing TiC particles dispersed therein which is similar to that disclosed in U.S. Pat. No. 4,731,115 cited above. This alloy material is characterized by that the matrix alloy is of .beta. phase. It claims that the titanium alloy material, in which the matrix is of .beta. phase, is by far superior in wear resistance to that in which the matrix is the ordinary .alpha.+.beta. titanium alloy.
The composite material containing TiC particles dispersed therein, which is disclosed in Japanese Patent Laid-open No. 129330/1990 cited above, has both improved wear resistance and improved ductility because it has the matrix of .beta.-titanium alloy. Nevertheless, it has a disadvantage of high production cost. It has an additional disadvantage inherent in .beta.-titanium alloy. A .beta.-titanium alloy has a much lower Young's modulus than an .alpha.+.beta. titanium alloy and hence it has the same stiffness as that of an ordinary .alpha.+.beta. titanium alloy even though it contains reinforcing particles dispersed therein. Also, a .beta.-titanium alloy is inherently poor in creep characteristics and hence it is poor in heat resistance even though it is incorporated with reinforcing particles.
U.S. Pat. No. 4,968,348 discloses "Titanium diboride/titanium alloy metal matrix microcomposite and process for powder metal cladding". According to this disclosure, the titanium-based composite material and powder alloy laminate are produced from a titanium alloy containing TiB.sub.2 particles dispersed therein which is prepared by powder metallurgy similar to that disclosed in U.S. Pat. No. 4,731,155 cited above. The thus obtained alloy composite material is claimed to be superior in strength, stiffness, and wear resistance. A disadvantage of this composite material is that the production process involves sintering at a low temperature under a high pressure because TiB.sub.2 is not in thermodynamic equilibrium with the titanium alloy. This limitation leads to a high production cost.